1. Field of the Invention
The present invention relates to a gyro for detecting an inertial angular rate of an object and more particularly to cyclically driven gyro which cyclically vibrates the mass in a vibration or rotation system.
2. Description of the Prior Art
The most traditional gyro apparatus is the type that detects a torque which acts in proportion to the vector product of an angular momentum of a rotor and an imputted angular rate around an axis perpendicular to the rotary axis of the rotor. Since this type of apparatus uses a complicated mechanism for detecting the torque which acts upon the supporting shaft of the rotor while rotating the rotor and is required to have a high precision, there are drawbacks such that the apparatus has to be made large in size and becomes expensive.
On the other hand, there has been proposed recently a system that cyclically drives a mass in the form of vibration or rotation and detects the Coriolis force which acts in proportion to the vector product of its driving rate and an input angular rate. In this system, a demodulation means is required to modulate the input angular rate by the cyclically driven vibration. However, compared with the traditional system using a rotating rotor, a drastic simplification in mechanism can be expected, so that it becomes possible to realize a reduction in size and cost.
As a cyclically driven gyro, there have so far been available a type that electromagnetically detects a deflection caused by a Coriolis force acting at the foremost end of an electromagnetically vibrated cantilever when an inertial angular rate is inputted, a type that utilizes piezoelectric effect for either one or both of the excitation and the detection (U.S. Pat. No. 2,544,646), a type that picks up a Coriolis force acting on a cyclically driven cantilever form piezoelectric element (U.S. Pat. No. 2,716,893), etc.
As one of the problems in such apparatuses, there is a response to a disturbance inertia. That is to say, a disturbance inertia when applied to the apparatus acts upon the above mass element to provide an error output.
Accordingly, the key point to constitute the apparatus is how to separate the component proportional to the Coriolis force and the component proportional to disturbance inertia from the deflection detection signal which is a response to the force which acts upon the mass element. In this case, since the Coriolis force is proportional to the vector product of the input inertial angular rate and the driving rate, the Coriolis force which acts on the mass element acts in the direction crossing at right angles with both of the instantaneous driving rate and the input inertial angular rate.
Therefore, a cyclically driven gyro may be constructed such that a pair of mass elements are driven symmetrically in reverse directions to each other and the outputs obtained by detecting the forces acting in the reverse directions are added so that the component proportional to the Coriolis force is added and the component proportional to the disturbance inertia is cancelled. As the result, it becomes possible to reduce the error response to the disturbance inertia.
In this case, the extent of the cancellation effect is determined by the balance of the sensitivities to the forces acting upon the two mass elements. That is to say, if the respective sensitivities are fully balanced, the error response to the disturbance inertia becomes zero, but if there is an imbalance between the two, there occurs an error response which is proportional thereto. When the cancellation effect at this time is compared with the case where the mass element and the detection means are independently provided, in the case that a 10% imbalance exists, the cancellation effect becomes approximately 1/20 because the component proportional to the Coriolis force becomes about twofold while the component proportional to the disturbance inertia becomes about 1/10, and in case of 1% imbalance, the cancellation effect becomes approximately 1/200 likewise.
Accordingly, when it is desired to suppress the response to the disturbance inertia, the imbalance must be reduced.
The effort made to reduce the imbalance in the conventional gyro of this kind was solely to improve symmetricity in the constituting elements. That is to say, attempt has been made to equalize the settings of the mass elements as well as the pickups for detecting the forces acting thereon as precisely as possible.
However, to seek a high precision for each of the constituting elements in this manner inevitably leads to a rise in product cost, reduction in yield, etc. The balance that would be attained even with such sacrifices is limited; it is difficult to obtain the desired characteristics; and it may, in some cases, be necessary to provide a very delicate arrangement which is very difficult.